avcodec/dnxhddec: ask for samples with alpha

libavcodec/dnxhddec.c

@@ -67,6 +67,8 @@ typedef struct DNXHDContext {
     const CIDEntry *cid_table;
     int bit_depth; // 8, 10, 12 or 0 if not initialized at all.
     int is_444;
+    int alpha;
+    int lla;
     int mbaff;
     int act;
     int (*decode_dct_block)(const struct DNXHDContext *ctx,
@@ -205,6 +207,10 @@ static int dnxhd_decode_header(DNXHDContext *ctx, AVFrame *frame,
         ctx->cur_field = 0;
     }
     ctx->mbaff = (buf[0x6] >> 5) & 1;
+    ctx->alpha = buf[0x7] & 1;
+    ctx->lla   = (buf[0x7] >> 1) & 1;
+    if (ctx->alpha)
+        avpriv_request_sample(ctx->avctx, "alpha");
 
     ctx->height = AV_RB16(buf + 0x18);
     ctx->width  = AV_RB16(buf + 0x1a);